The WhereToSearch part of your vlookup formula is being told to look in a place that it can’t find.
It could be that a cell that was previously being referenced is no longer there (has been deleted) or was never there. For example the formula might be looking in column 4 when you’ve only given it a 3 column table to look in
The formula below would return a #REF! and the 4 would need to be changed to 3 or 2.
=vlookup(“elucidate”, A:C, 4, false)
If you’re typing worksheet names in manually to the formula and it doesn’t match exactly the intended worksheet’s name, you’ll get a #REF!
Lastly, if the worksheet where the WhereToSearch part of your formula is referring to has been deleted since you created the formula… same again #REF!